1. Technical Field
The present disclosure is directed improving energy efficiency of HVAC components, and in particular, to improved apparatus and methods for controlling power factor correction in an HVAC variable speed motor drive which improves efficiency, reduces manufacturing costs, and increases reliability.
2. Background of Related Art
Many heating, ventilating and air conditioning (HVAC) components use variable-speed drive (VSD) systems that improve energy efficiency by matching motor speed to demand load. In contrast to single-speed systems, where, for example, an air conditioning or heat pump compressor drive motor is either stopped, or operating at full speed, in a variable speed system the compressor drive motor is driven at a speed varied in accordance with the amount of cooling or heating required.
The ability to tailor power output of a compressor, pump, or an air mover, rather than simple on-off operation, helps achieve significant energy and cost savings. In some cases, VSD systems consume up to 40% less energy when compared to single-speed systems of equal capacity. Additionally, VSD HVAC systems have benefits beyond energy efficiency. A variable-speed air conditioning system can provide better dehumidification when run at less than full capacity, since the increased cycle times that result from low-speed operation allow more moisture to be condensed out of the air before reaching the temperature set point. Variable speed system tend to run quieter at lower speeds, and since bearings and other mechanical components are subject to reduced wear during low speed operation, service intervals may be extended and reliability enhanced.
A variable speed drive system commonly employs a variable frequency drive (VFD) circuit that includes a nonlinear AC-DC power supply unit (PSU) and an inverter. The VFD utilizes the PSU to convert fixed-frequency AC from the utility (typically 50 Hz or 60 Hz) into DC, which is then converted by the inverter into variable-frequency AC to drive the motor. By varying the VFD output frequency, an induction or permanent magnet motor is driven at the desired speed determined by the drive frequency.
Non-linear AC-DC power supplies used in VFD circuits draw non-sinusoidal currents, in part because they use solid-state rectifiers at their inputs in conjunction with filter capacitors, which alters the phase relationship between current and voltage delivered by the utility. These non-sinusoidal currents and altered phase relationships cause the power supply to exhibit poor power factor performance and undesirable harmonic emissions. To address this, such power supplies may include a passive or active power factor correction (PFC) circuit designed to improve power factor performance. Known PFC circuits may have drawbacks. Passive PFC circuits may increase efficiency with a lower power factor, while active PFC circuits tend to have lower efficiency with higher power factor. In addition, known passive PFC arrangements require more capacitance, which add expense, weight, and bulk.
Governing bodies have enacted performance standards with which HVAC units must comply. For example, newly-manufactured HVAC units are subject to energy efficiency standards, such as the Seasonal Energy Efficiency Ratio (SEER) testing standards. The SEER rating attempts to characterize the efficiency of an HVAC system as a single number which is determined by testing the system under a series of precisely-defined load and environmental conditions, known as test points. In another example, HVAC equipment may be subject to standards, such as IEC 61000-3-2, which govern power factor performance and/or harmonic emissions. A VFD system that improves efficiency and power factor performance in a cost-effective manner would be a welcome advance.